1. Technical Field
The present invention relates to a composition that cures at room temperature to give silicone elastomer. More specifically, the present invention relates to a composition that cures at room temperature to give silicone elastomer whose postcure physical properties can be varied over a broad range.
2. Prior Art and Problems Therein
Compositions that cure at room temperature to give silicone elastomer are already known and have already entered into use in a wide range of industrial sectors. The mechanisms underlying their room-temperature cure include the hydrosilylation reaction, ultraviolet-based curing, and curing based on the condensation reaction between the silanol group and silicon-bonded functional groups. Among these, silicone elastomer compositions that cure by a condensation reaction mechanism offer the following features ready development of adhesiveness at room temperature, resistance to cure inhibition by contaminants that might be present in the curing environment, brief cure time obtained by simply mixing the base and catalyst, long-term storage stability as the single-package composition, and development of cure by simply standing in the atmosphere. As a result of these features, such silicone elastomer compositions have entered into broad use as adhesives, coatings, and sealants.
However, a problem associated with these particular compositions is that limitations are imposed on their post-cure mechanical properties as a result of the requirement that they exhibit pre-cure workability, i.e., in mixing, placement, and finishing by manual procedures. In specific terms, because the molecular weight of the polydiorganosiloxane main component must be held below a certain level in order to facilitate workability, it becomes essentially impossible to reduce the elastomer's postcure stiffness (as represented by such properties as the Durometer hardness and modulus) below a certain level.
The simplest method for solving this problem consists of the addition of unreactive polydiorganosiloxane. The problem with this method is that the unreactive polydiorganosiloxane additive bleeds onto the surface after curing and impairs the adhesiveness.
A fundamental strategy for solving this problem consists of the use of both polyfunctional crosslinker and difunctional chain extender. The polydiorganosiloxane then undergoes both crosslinking and chain extension during the crosslinking reaction and the post-cure crosslink density is thereby reduced.
The following two methods have been proposed within the sphere of this fundamental strategy:
(1) the addition of siloxane bearing two N,N-dialkylaminoxy groups in each molecule and siloxane bearing at least three N,N-dialkylaminoxy groups in each molecule, and PA0 (2) the addition of silane bearing two N-alkylacetamido groups in each molecule and either silane bearing at least three N-alkylacetamido groups in each molecule or siloxane bearing at least three N,N-dialkylaminoxy groups in each molecule.
Nevertheless, these methods still suffer from problems.
In the case of the first method, the use of the N,N-dialkylaminoxy group results in the generation of N,N-dialkylhydroxylamine by-product by the curing reaction. This creates the problem of an unpleasant hydroxylamine odor. In addition, as a result of the strong basicity of hydroxylamines, even a slight elevation in ambient temperature results in the critical problem of cure inhibition due to polydiorganosiloxane scission. Finally, this method is economically disadvantageous because N,N-dialkylaminoxy-containing siloxane is expensive.
In the case of the second method, the use of the N-alkylacetamido group again creates an odor problem due to the N-alkylacetamide produced as by-product during cure. Another drawback to the use of the N-alkylacetamido group is the occurrence of substitution reactions to give, e.g., the alkoxy group, when an active hydrogen-containing compound, e.g., alcohol, is present in the ambient. This also causes cure inhibition. Finally, this method is also rendered economically disadvantageous by the expense of N-alkylacetamido-containing silane.
It has otherwise been proposed that chain extension and crosslinking be conducted using functional groups that are already in wide application and that do not cause secondary reactions. This approach avoids the use of special, expensive functional groups as are used in the two methods discussed above. Japanese Patent Application Laid Open [Kokai or Unexamined] Number Sho 63-83167 [83,167/1988], assigned to Toray Silicone KK, proposes a method that uses, for example, RNHCH.sub.2 MeSi(OMe).sub.2, as chain extender. However, this method cannot be applied at a practical level because it is extremely difficult to economically synthesize this chain extender and because it is difficult to strike a stable balance with the crosslinker.